Vacuum assist pumps are provided for assist brake in most cars and light vehicles, in which the magnitude of brake power assist generated by the vacuum assist pump has close relation to the rotation speed of an engine. When a brake pedal is pressed down, a diaphragm within the vacuum assist bump is drawn by means of the engine's air suction so that one push rod is caused to engage on the brake pump, and thus the braking force increases.
It is known from the Chinese patent ZL02 2 82639.4 a coasting control device for a vehicle adopting an overrunning clutch. Its principle is as follows: when the vehicle decelerates, it is out of the engine damping via overrunning clutch in order to coast a longer distance without the engine damping, and thus the purpose for saving fuel is attained. However, there is a serious safety problem in this technical scheme, because the vehicle is nothing more than in the neutral coasting state as the coasting control device is operated and the suction ability is very low as the engine runs in idling state, so that the vacuum assist bump can not reach its expected brake power assist, and the braking performance decreases, which is not allowable in the vehicle driving criterion and the vehicle manufacture criterion.